"If Stoneplate/Cho Ult is such a problem, why wasn't it meta before his small rework?"
Because you're stupid, that's why. It was a problem, but he wasn't popular and as a result, he wasn't played enough for it to have a considerable sample size to act as sufficient evidence to prove it. Remember AP Trynd in season 3? Nothing really changed that suddenly made him good, it was a relatively unknown playstyle people weren't aware of after some changes to him. I believe Morello himself even stated they always suspected AP Trynd could be overpowers, but it simply wasn't popular enough to grant quality sample sizes to make such a change.
Moreover, his E gave him something he previously didn't have: reliability and perpetual sticking power. Previously, Cho had no means of sticking to you when missing rupture. Now, he Es you and you're never getting out of his range again. That's well and fine, until he activates Stoneplate and does 2k true damage. I don't have a problem with Cho having consistent sticking power, decent DPS and even a nice true damage nuke - but not at 2k values from an interaction with a tank item. There's really no debating this point alone: this interaction is fucking broken and doesn't belong in the game.
Lastly, rarely do direct additions or changes result in a feature or interaction instantly being broken. It typically takes a few indirect changes to pave the way for that entity to become constant. Truthfully, I'd argue this is the foundation of overpowered independent of raw statistical overtuning: something that performs far too well too consistently while ignoring its weaknesses. This is what paved the way for Cho'Gath to go from a weak power leve to an absurd one.
Removing Stoneplates interaction with Cho R is the best way to balance him because it truly is the only abusive feature he has and it needs to go. I do not understand why the people on this board defend such a ridiculous feature. It's brain numbing.

or upcoming
). then of course they can buff him if (big if) they need to after the change, as even with the ar-pen, it would still be weaker than the true damage version that is live.